> The other point to bear in mind is that we won't know whether there is a limit to quantum entanglement until we encounter it.

That’s true, but...

> There is no law of physics that states we must be able to form arbitrarily complex superpositions of quantum states. That is an assumption in our current models of physics.

That’s not quite true. (Or perhaps it’s more correct to say that it’s true, but misleading.) It’s true in the same sense that the speed of light being a constant is the same in all reference frames is an “assumption.” The evidence that this is correct is overwhelming, and the evidence that quantum mechanics applies to arbitrarily large systems of entangled states is likewise overwhelming. All attempts to find an experimental regime where quantum mechanics fails have failed. If you seek solace against the possibility of cryptographic keys falling to quantum computers you are better off looking for it in engineering constraints than in the prospect of discovering new physics.

"All attempts to find an experimental regime where quantum mechanics fails have failed"

That is not remotely true. We still haven't figured out how to make gravity work. More relevantly, what exactly do we mean by decoherence? what exactly is it that causes it?

The assumption here seems to be that the only reason to build quantum computers is to break RSA. That is not the case. Quantum simulation is the real prize and what I am saying is that we might well find that there is some effect that imposes a limit there.

We have yet to see a situation where investing $x.Y produces a quantum computer that is more than x times faster than investing $Y in a quantum computer. It is not just the number of QBits that is at issue here, it is the number of operations you can perform before you lose coherence.

People should recognize that quantum computers are a cheap way to do experiments in fundamental physics at this point. This is science, not engineering. I used to do experimental particle physics so it is science I am somewhat familiar with. But it is still not engineering and unlikely to be for another decade or two if ever.